MIMO technology enables a high spectral efficiency to be obtained. This high spectral efficiency, measured in terms of bits per second per Hertz, has made MIMO technology attractive for many communication standards such as Long Term Evolution (LTE), LTE-Advanced, the Wireless Local Area Network (WLAN) standards Institution of Electrical and Electronic Engineers (IEEE) 802.11n, and WiMax, also known as IEEE 802.16e, for implementing the high data rate transmission in the range 100 Mbit·s−1 to 1 Gbit·s−1. In a communication system employing MIMO technology, data to be transmitted is divided into a plurality of data streams, and different data streams are transmitted from different transmit antennas at the same time in the same frequency band. The plurality of data streams intentionally interfere with each other, and a receiver for receiving the plurality of data streams has to separate the data streams from each other. The data streams comprise symbols, where a symbol is a signal representative of an integer number of binary digits, that is, bits. Typically, each symbol is representative of a plurality of bits.
An element of such a MIMO receiver that affects reception performance is an equaliser. The equaliser detects the plurality of received data streams and mitigates the effects of inter-symbol interference between different symbols transmitted simultaneously on the different data streams. A channel decoder, such as a turbo decoder, can then be used to recover bits of the transmitted data from the equalised symbols.
In a MIMO system with M transmit and N receive antennas, the received signal y can be expressed as an N dimensional received signal vector,y=Cs+n  (1)where s is an M-dimensional vector of symbols transmitted simultaneously from each of the M transmit antennas, C is a channel matrix with N rows and M columns, and n is an N-dimensional additive complex Gaussian noise vector. For the case of two transmit antennas, s=[s1, s2], where s1 is a first transmitted signal transmitted from a first one of the transmit antennas, and s2 is a second transmitted signal transmitted from a second one of the transmit antennas. Also, for the case of two receive antennas, y=[y11, y21], where y11 is the signal received at a first one of the receive antennas, and y21 is the signal received at a second one of the receive antennas. The symbols of s are chosen independently from a complex signal constellation of, for example, Quadrature Phase Shift Key (QPSK) modulation, 16-Quadrature Amplitude Modulation (16-QAM), or 64-Quadrature Amplitude Modulation (64-QAM). The symbols may be represented by complex numbers, each mapped to a unique bit pattern. For example, for QPSK modulation, a signal constellation of four symbols may be represented by the set of complex numbers {−1−i, −1+i, 1−i, 1+i}.
FIG. 1 illustrates a block schematic diagram of an example MIMO receiver 100. Referring to FIG. 1, the MIMO receiver 100 comprises a first receive antenna 102 and a second receive antenna 104, both of which are coupled to a demodulator 110 for demodulating signals received at the first and second receive antennas 102, 104. The signals received at the first and second receive antennas 102, 104 comprise the plurality of data streams transmitted simultaneously from different transmit antennas. The demodulated received signals are delivered to an input of a MIMO equaliser 130. A channel estimator 120 is coupled to the demodulator 110 and estimates, from the received signals, the characteristics of the communication channel between the transmit antennas and the receive antennas 102, 104. The channel estimator 120 delivers the estimated channel characteristics to the MIMO equaliser 130, and the MIMO equaliser 130 employs the estimated channel characteristics in detecting the symbols of the transmitted data streams. Each detected symbol comprises soft bits values, and the soft information may be provided as log-likelihood ratios (LLR). The detected symbols are de-scrambled by a de-scrambler 140 and de-interleaved in a de-interleaver 150, and the de-scrambled and de-interleaved symbols are decoded by a turbo decoder 160. The turbo decoder 160 delivers decoded bits at an output 162 of the MIMO receiver 100.
There is a requirement for improvements in receivers for use in MIMO systems.